Fracture strength evaluation of ultra-thin glass by ball-on-ring test considering kinematic nonlinearity

Abstract

Ultra-Thin Glass, UTG is a glass with thickness under 100 μm. Due to its superior property such as high transmissivity, extremely low surface roughness and low flexural rigidity it has been anticipated as one of the best candidate materials for advanced electronics substrate and encapsulation layer. However due to the brittle nature of glass, reliability issues must be overcome for application of UTG. Especially when exposed impact, high tensile stress is concentrated instantaneously at a small region which results fracture in UTG. Therefore, it is essential to evaluate mechanical strength of UTG and use it as a guideline with safety factor for design of UTG integrated advanced electronics. In this study Ball-on-Ring test was set up to measure mechanical strength of UTG excluding the effect of edge and sidewall defect. Analytical Ball-on-Ring test strength calculation equation isn’t applicable to UTG, since the equation was derived under small displacement assumption which result in linear relation between load and maximum tensile stress. Displacement result of UTG Ball-on-Ring test is greater than specimen thickness which makes small displacement assumption invalid. Due to this large displacement kinematic nonlinearity prevails resulting nonlinear relation between load, stress and displacement. Therefore, finite element analysis considering nonlinearity was conducted to calculate UTG strength. UTG strength calculated by Ball-on-Ring test and empirical equation decreases as the thickness of UTG increases. According to weakest link theory brittle material’s strength is dependent of effective volume at which tensile stress is imposed. As thickness of UTG increases calculated effective volume increases resulting lower UTG strength. From finite element analysis result thickness and elastic modulus were parameters affecting nonlinear relation, by considering these factors an empirical equation is suggested which enables facile calculation of UTG strength without use of finite element analysis. Measured UTG strength values and their thickness dependency by Ball-on-Ring test and empirical equation are expected to be used as a guideline for designing UTG integrated device.